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Hmmmm, I'm not familiar with keeping the back leg "straight and locked" during Aikido movements. I imagine it would be difficult to have smooth and fluid movement if one of your legs was straight (instead of slightly bending at the knees). Perhaps the concept of "straight and locked" only applies during certain standing pins or maybe the particular style of Aikido you study concentrates more on this type of movement. I've attended several Yoshinkan Aikido seminars and have noticed that many of the final movements during a technique do resemble a stance with the back leg being "straight and locked". I was told that this enables Nage (Tori) to better extend their energy through Uke by having a more stable center. Other than that, I would say ask your Sensei as I'm sure he / she can give you a much better reason. Have a good day!

You might have it straight so that you can redirect force applied against you into the ground.

andrew

This is one reason for keeping the back leg straight, for support like Andrew stated.

Another reason to keep the back leg straight is to add power to your throws. The hips need that solid bracing to transfer as much power as possilbe to uke. I'd also add here that the heel of the back leg must stay planted against the ground thru the entire throw. If the heel raises, and you execute the throw with the back foot up on the ball of the toes, you lose much of the power of the throw. Experiment with this and see for yourself!

Regards,

Brian Vickery

"The highest level of technique to achieve is that of having NO technique!"

I think one reason for extending your rear leg when pivoting or throwing is to maximize the rotation of your hips, which is where alot of the power for such techniques comes from. This is consistent not only with aikido, but with other martial arts as well.

In japanese they use the word funbaru to describe this stiff back leg. I look at it from a sword perspective. A sword cut is a round thing. you throw the sword out hack into the persons head a bit, then pull down and back towards your center, effectively, *slicing.*
So to recap, a katana cuts with it's own weight and the cutting motion not with the power of the upper body. The better you get, the less the upper body is used, and the more one throws it out and then squeezes their tummy to pull it back in. It is a lot like fishing.
To connect this to aikido, most throws or extensions are very similar to cutting. When that wait gets pulled back into your center, all the energy has to go somewhere. If your back leg isn't straight, the energy comes into your center and disrupts it. It breaks your center. Try swinging a sledge hammer without losing your balance. Now one more time, straighten the leg, and sink the hips until everything in your lower body is *taught* not stiff perhaps, but filled with enough tension as to effectively conduct the energy of the strike not into your center but through center to the center of the earth. A good iaido cut is really big and really fast, and the person cutting appears to move nothing but there arms. A good throw is the same. Minimal steps, and minimal reaction just a big mountain throwing around a rag doll attacker. This is someone with a powerful lower body! Everytime you throw you get carried along with, or you find your posture being bent; this is a powerful upperbody.
Funbaro!

Thomas, you study Iaido right, well in kenjutsu that's not how we would cut. You may let the katana do the cutting for you, and that might get through the skin but it not getting through a skull, and never a helmet. You have to drop your weight down to cut, not slice and it should not be a big wide arc it should be direct and straight to the point. I have no problem using my kenjutsu techniques against the guy at Aikido who do Iai exactly because of the way you cut. Especially Tsuri Age (push up), you can really get a lot of push up on that big arc and that leaves your 'do' (to use a kendo phase) open for a long time. Other great techniques that are effective against Iai's slice are morote giri (two hand cut) and kote giri (wrist cut) (I think in kendo they call it ura or gyaku kote giri, any way it's cutting the bottom of the wrist), you give people a lot of time to get under your arms. The few techniques I mentioned all work by getting in under the arms and against Iaidoka I haven't had any problems using them.
Any way back to Aikido, my kenjutsu has taught me to cut with my centre, so I throw with my centre. So if any power (for lack of a better word) come back it goes down through both legs via my centre.
Any way just my opinion and interpretation.

PS This is not aimed at you Thomas, wherever it says 'you' I mean Iaidoka in general.
I am just trying to say Iai is not to combat oriented don't get me wrong its a great art and the demonstrations I have seen are pretty sweet.